Electromigration (EM) failure remains one of the most challenging problems facing the semiconductor industry. EM is the process by which a large electrical current flowing through a fine conducting wire or element can cause atomic diffusion in the direction of the electron flow. This phenomenon is thermally activated, and becomes exponentially stronger at higher temperatures, as it depends as a power-law on the current density. EM is one of the most common failure mechanisms in integrated circuits (IC), which operate at high current densities (approximately 100 kA/cm2) and elevated temperatures (greater than 50° C.). Physically, the momentum transfer between electrons and the atomic lattice causes atoms to diffuse, creating vacancies or “voids” in the fine wire. These voids can then move, combine, and divide, exhibiting surprisingly complex dynamics that is not well understood. The process of diffusion eventually causes the fine wire to break and fail. Even a small number of EM-induced defects may cause an IC to cease to function. Future ICs promise to operate with higher current densities and higher operating temperatures, as the industry trend toward smaller conductors continues to progress. As this trend continues, the significance of the EM failure mechanism will continue to grow.
EM is notoriously complex and depends on a large number of factors: the composition, microstructure, and dimensions of the wires, the ambient operating conditions, the thickness and composition of any overlayer, etc. Under typical operating conditions, EM can take months or years to become a significant problem in IC operation. Therefore, virtually all EM testing is conducted under accelerated testing conditions, which feature higher temperatures (>150° C.) and much higher current densities (0.5-5 MA/cm2) than are found in typical operational conditions for ICs. The results can then be extrapolated to more typical operating parameters. Because EM is such a difficult problem, with such far-reaching impacts on IC fabrication, there are many groups in both academia and industry attempting to understand this phenomenon.
Methods for Studying Electromigration
Currently there are several techniques for studying EM, which include mean-time-to-failure measurements, resistometric evaluations, noise measurements, and microscopy. Unfortunately, each of these techniques fails to adequately describe EM, as described below.
Mean-time-to-failure (MTF) measurements provide a quantitative measure of IC resistance to EM, but offer no insight into the actual dynamics of the EM process. These measurements stress many conductors until a certain failure condition is achieved. Once failure conditions are achieved, statistical analyses provide insight into predicting IC failure from EM.
Both resistometric evaluations and noise measurement techniques give precise information about the electrical properties of conductors at various points in time, but the results of such tests are often difficult to interpret, and cannot provide any information about the micro-structural changes occurring in the sample.
Microscopy techniques have used both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM provide microstructural imaging of samples damaged by EM. Due to sample preparation requirements, both SEM and TEM techniques are destructive and can only be used on samples not “buried” by a dielectric overlayer. The value of SEM and TEM is thus limited because all ICs include a passivation layer to protect the active layer(s), and this passivation has a critical influence on the EM process. In addition, these techniques can require elaborate and time-consuming sample preparation procedures, such as the use of a focused ion beam (FIB) to cleave samples for TEM studies. Another drawback to these techniques is that they provide little quantitative information about the thickness of EM-induced voiding.
Other technologies presently available have been unfulfilling to date in efforts to gain more information about EM.
Magnetic Sensing Technologies
A physical object may generate a magnetic-field (H) that can be sensed near the surface of the object. Using magnetic sensing technology, an “image” of the magnetic field distribution may be obtained. Such images can be spatially microscopic and weak in field strength. Nevertheless, these images reveal important signatures of inherent electrical and magnetic processes within the objects. For example, the magnetic image of a magnetic thin film discloses its internal magnetic domain structure. The electrical currents inside an IC chip generate external magnetic fields, which not only contain information about the spatial variation of current density, but also the frequencies with which various components on a chip are operating. A type II superconductor also creates threading magnetic flux lines that may be imaged, whose structure and dynamics reveal fundamental properties.
There are various techniques presently used to detect magnetic properties at a surface of a sample. These have included electron holography, scanning electron microscopy with polarization analysis (SEMPA), magneto-optical microscopy, and scanning magnetic microscopy (SMM).
Electron holography and SEMPA require high vacuum operation and delicate sample preparation. Both of these techniques offer static field images with good spatial resolution. However, the instruments are expensive and demand great technical skill to operate. The magneto-optical microscope is a relatively simple system and suitable for time-resolved imaging. However, its field sensitivity and spatial resolution are poor. It should be noted that many of these techniques are sensitive to magnetization, rather than stray field, which makes them unsuitable to the application at hand.
Scanning magnetic microscopes have a magnetic sensing element, such as a SQUID or Hall effect element, which is physically scanned relative to a sample to obtain a local field image. Though very sensitive, a SQUID probe is poor in resolution (˜30 μm), and requires cryogenics to operate. A Hall probe can operate under ambient conditions, but its sensitivity is low. Microscopes equipped with a magnetic tip, using a technique called magnetic force microscopy (MFM), can only measure the gradient of a magnetic field, and have not yet demonstrated their usefulness for application to current imaging.
The result is that no successful technique for high resolution, time-lapse or real time EM imaging is commercially available. The trend towards reduced IC feature size and a large number of metal layers mandates the development of a commercially viable technique that is suitable for EM imaging.